1. Field of the Invention
This invention relates to an optical transmitting device, an optical receiving device or a unified optical transmitting/receiving device.
This application claims the priority of Japanese Patent Application No. 2000-31891 filed on Feb. 9, 2000 which is incorporated herein by reference.
The devices are called an optical communication device as a whole. Namely, the present invention includes three sorts of concrete devices,    1. optical transmitting device,    2. optical receiving device, and    3. optical transmitting/receiving device.This invention can be applied to all the three kinds of the optical communication devices.
This invention aims at an improvement of the optical communication devices for finding disorders by observing the reflection of monitoring light from the devices. The present invention aims at providing, in particular, a low-cost, small-sized optical transmission or optical receiving devices suitable for the subscriber systems or the ONU (optical network unit) systems. The contrivance of reflecting monitoring light is installed to the optical communication devices for finding a break of the fibers which connect the unit to other units. The distinction of the reflecting light from the fiber notifies an occurrence of a trouble of a fiber-break. This invention aims at an inexpensive monitoring contrivance which would not raise the cost of the ONU module so much:
2. Description of Related Art
In prior optical communication systems, optical transmission devices, optical receiving devices or optical transmitting/receiving devices have been constructed by assembling the LD module (prior art) of FIG. 1 or the PD module (prior art) as shown in FIG. 2. The LD module of FIG. 1 hermetically seals an LD chip in a metal can package having a three-dimensional structure. The external shell portion is built with a metal stem 1, a cylindrical metal lens holder 2 and a conical metal ferrule holder 3. The shell forms a firm structure in which the parts are bound by welding. A cylindrical cap 4 having an opening is fixed upon the stem 1 in the package.
An LD chip 6 is mounted upon a side of a pole 5 erected on the stem 1. A PD chip 7 for monitoring the power of the LD 6 is furnished just beneath the LD chip 6 upon the stem 1. A lens 8 for converging the LD light is held at the upper opening of the lens holder 2. The ferrule holder 3 has a vertical narrow hole which maintains a ferrule 10. The ferrule 10 keeps an end of an optical fiber 9. The ends of the ferrule 10 and the fiber 9 are polished slantingly for preventing the light reflected at the fiber end from returning into the LD chip 6. The round stem 1 has a set of lead pins 11 which are input or output terminals of sending signals or monitoring signals.
The LD chip 6 emits forward signal light (sending light) including sending information. The signal light is converged by the lens 8 and is guided into the optical fiber 9. The signal light propagates in the fiber 9 to another unit. The LD module which is hermetically sealed by the metal package has high reliability. Such a metal-can type LD module has been widely employed in optical information systems. However, the metal sealed module is expensive due to high parts cost and high assembling cost. The high cost derives from alignments for seeking the optimum positions of the LD chip 6, the lens 8 and the fiber 9 by displacing the stem 1, the lens holder 2, the ferrule holder 3 and the ferrule 10 three-dimensionally in the X, Y and Z-directions and measuring the output power at the other end of the fiber 9. The alignment takes a long time. The alignment enhances the cost of the LD module of FIG. 1.
FIG. 2 shows a prior art PD module. The PD module has also a three-dimensional structure like the LD module of FIG. 1. The PD module has a round metallic stem 12, a circular metallic lens holder 13 and a conical metal ferrule holder 14 as a metallic package. In the inner space of the package, a round cap 15 with a top opening is fixed to the stem 1. The stem 12 has a PD chip 16 at the center. A lens 17 is kept in the top opening of the lens holder 13 just above the PD chip 16. A vertical top hole of the ferrule holder 14 sustains a ferrule 19 which seizes an end of an optical fiber 18. The prior PD module also requires three dimensional alignments including two dimensional alignments among the stem 12, the lens holder 13 and the ferrule holder 14 in the planar directions and a one dimensional alignment of the ferrule 19 in the axial direction. At present, the PD module of FIG. 2 and the LD module of FIG. 1 having the three-dimensional structure are widely employed in the optical communication systems.
FIG. 3 schematically shows a two-fiber type optical communication system. A central station has an LD1 and a PD1. A subscriber (ONU) has an LD2 and a PD2. The central station sends signals from the LD1 to the subscriber PD2 through an optical fiber 21. The signals from the station to the ONU are called downstream signals. The ONU sends signals from the LD2 to the central station through another optical fiber 22. The signals are called upstream signals. The fiber 21 is inherent to the downstream signals. The fiber 22 is an exclusive medium for the upstream signals. The upstream signals and the downstream signals propagate in the different fibers 22 and 21. The two fibers 21 and 22 enable the light of a single frequency to transmit signals in two directions simultaneously. The advantage of the two fiber type is the probability of realizing the simultaneous bidirectional transmission with the single frequency light.
Such a two-fiber type optical communication system was suggested, for example, by;                {circle around (1)}R. Takahashi, K. Murakami, Y. Sunaga, T. Tokoro, M. Kobayashi, “Packaging of optical semiconductor chips for SFF optical transceiver”, PROCEEDINGS OF THE 1999 ELECTRONICS SOCIETY CONFERENCE OF IEICE, C-3–28, p 133(1999).This report discussed the problem of the optical crosstalk but says nothing about the detection of disorder.        
FIG. 4 shows a single-fiber type optical communication system which transmits signal light in two directions in a single fiber. A central station has an LD1 and a PD1. An ONU has an LD2 and a PD2. The central station and the subscriber are connected by a single optical fiber 24. This system can make use of the light of the same frequency when the signals are transmitted in both directions at different times (ping-pong transmission), since no cross talk happens between the signals which are transmitted in two directions. However, in the case of the simultaneous bidirectional transmission, two different wavelengths λ1 and λ2 of signal light should be required for separating the bidirectional signals. The upward signal should ride on a first wavelength λ1 and the downward signal should be carried by a second wavelength λ2. For separating two wavelengths, wavelength division multiplexers 23 and 25 should be placed at the both ends of the optical fiber 24. The wavelength division multiplexers 23 and 25 discriminate the downward signal λ2 from the upward signal λ1.
For example, the bidirectional system employs an optical transmission/reception apparatus as shown in FIG. 5 for the ONU terminal. The ONU has an LD module 26 and a PD module 27. Both modules of the LD and the PD are independent, as shown in FIG. 1 and FIG. 2. The LD module 26 makes upward signals of λ1. The upward signals λ1 pass an optical fiber 28, an optical connector 29, an optical fiber 30 and go into a WDM 31 on the ONU. The WDM 31 has a kind of coupler with wavelength selectivity. The sending signals λ1 pass a fiber 30 in the WDM, an optical connector 34 and propagates in a fiber 35. The fiber 35 communicates with the central station. The downward signals λ2 from the central station to the ONU terminal propagate in the fibers 35 and 30 and enter the WDM 31. The WDM 31 changes the path of λ2 from the fiber 30 into a fiber 32. The receiving signal light λ2 passes a connector 37, goes in a fiber 36 and enters the PD module 27. The WDMs and the different frequency light enable the system to carry out the simultaneous bidirectional optical communication.
The prior art of the optical communication has been surveyed. The systems have been equipped with an apparatus of detecting a trouble of fiber break. For example, as shown in FIG. 6, the trouble detecting apparatus includes an extra light source which can emit light of a third frequency λ3 that is different from λ1 and λ2 at a central station 40 and a selective reflection device 45 which only reflects the λ3 light at the ONU. Sometimes the central station 40 sends the λ3 detection light to the ONU for detecting an occurrence of a fiber break trouble. If the fiber is normal, the detecting light λ3 is reflected by the reflection device 45 at the ONU and returns to the central station 40. If the fiber breaks at a spot, the detecting light λ3 is not reflected by the reflection device 45 at the ONU and does not return to the central station 40. The existence of the returning λ3 light confirms the normal fiber without break. The extinction of the returning λ3 light means an occurrence of the fiber break.
For example, FIG. 6 shows an optical subscriber system having a ratio of 1:N (N=16, 32 or so). Here, “1” means a fiber from the central station 40. “N” means the number of the subscribers connected to the central station through the same fiber. The central station 40 sends a downward signal λ2 via a fiber 41. The downward signal is divided by a 1:N divider 42 into N (e.g., N=16) subscribers through N fibers 43, 46. An ONU LD/PD module 47 is installed in a house 44 of the subscriber. The in-house LD/PD module 47 senses the λ2 signal. The LD/PD module 47 in the house is an apparatus having the LD module and the PD module as shown in FIG. 5.
The subscriber on an ONU transmits inherent upward signals via the fibers 43 and 41 to the central station 40 by λ1 light, e.g., 1.3 μm band light. The central station 40 receives the upward signals from the sixteen subscribers. Furthermore, the system has a fiber-break detection device which investigates whether the fiber network is normal or abnormal. The fiber-break detection device includes a detection light generator in the central station 40 and detection light reflectors 45 provided at outer walls of the houses 44 of the ONUs.
The detection light wavelength λ3 is different from both the downward light λ2 and the upward light λ1 which transmit signals. For example, the detection light λ3 is 1.65 μm light. The central station 40 sends the detection light λ3 to the ONUs of the subscribers. If the fiber is normal, the detection light λ3 is reflected at the detection light reflector 45 on the walls of the houses 44 and the returning light is sensed by the PD in the central station 40. The return of the detection light confirms the normality of the net work. If the fiber is broken at some spot, the detection light λ3 does not arrive at the detection light reflector 45 and is not sensed by the PD in the central station 40. If the returning light is feeble, some trouble should happen in the fibers. The fiber-break detection device examines the break of fibers, an increase of attenuation or detachment of connectors. This function is indispensable for public communication media.
The fiber-break detection device of FIG. 6 has a drawback that the detection light reflector 45 is just furnished at the interface between the indoor space and the outdoor space. The detection light reflector 45 should be designed, manufactured, sold and installed as an independent device. The installation requires the steps of cutting the optical fiber midway, inserting the detection light reflector into two ends of the cut fibers and joining the fibers to two terminals of the reflector. The independence of the reflector would require some pertinent housing for protecting the reflector, and nails, screws and nuts for installing the reflector at a wall of the house.
From the standpoint of economy, the installation of the detection light reflector invites new cost for detecting the fiber-break or other fiber disorder. The increase of cost is undesirable. Inexpensiveness is the most important matter for the optical communication to pervade wide in the general public.
Another problem is a technical one. Even if the fibers 41, 43 are all normal, there is still a possibility of the occurrence of the fiber-break or other disorders in an inner fiber 46 or connectors between the reflector 45 and the optical transmission/reception module 47 (LD/PD module) because of the detection light reflector 45 furnished at the interface. The fiber-break detection device of FIG. 6 cannot detect the occurrence of the inner fiber trouble happened in the indoor fiber 46 or the connectors. The detection system is not perfect yet. The fiber-break detection device of FIG. 6 is still an imperfect device. Being aware of the problem, someone proposes an improved detection light reflection device of adding a dielectric multilayer filter for reflecting only the detection light in the optical transmission/reception module 47 (LD/PD module). The extra dielectric multilayer filter allows the signal light to pass through without loss but reflects the detection light.                {circle around (2)}, Akira Morinaka, Yasuyuki Inoue, Kuniharu Katoh, Hiroshi Toba, Norio Takato, “Filter-type PLC-WDM Circuit with 1.65 μm cut/ref filter”, PROCEEDINGS OF THE 1998 IEICE GENERAL CONFERENCE, C-3-158, p 324(1998).        
The optical transmission/reception module proposed by {circle around (2)} has a Y-branched optical waveguide, a PD and an LD laid at the ends of the Y-branch for simultaneous bidirectional communication and a dielectric multilayer WDM interposed in the optical waveguide for reflecting the detection light. The WDM allows the detection light to enter the PD in the module. The reflection loss of the detection light by the WDM depends upon the inclination of the WDM to the path line. Inclination angles of 3 degrees to 5 degrees would give the WDM a 10 dB to 20 dB reflection loss for 1.65 μm (detection light). The PD would be disturbed by the detection light (1.65 μm) in this case. A pertinent increase of the inclination angle of the WDM would give the WDM a 50 dB reflection loss for 1.65 μm. The report alleged that the PD would not be perturbed at all by the detection light (1.65 μm) in the case of the large inclination angle.
Economical problems and technical difficulties still accompany such a monitoring device which installs the independent reflector at the interface between the indoor space and the outdoor space, as shown in FIG. 6. The device is not matured yet for practical use. The independent reflection device requires a high performance WDM filter for accomplishing high insertion loss for 1.65 μm light. Low yield of manufacturing the high performance WDM filter would raise the cost of the WDM. The high cost would enhance the parts cost of the monitoring device itself. The assembly cost would be also raised by the additional steps of making a narrow groove on a substrate and inserting the WDM filter into the narrow groove. One purpose of the present invention is to provide a low cost detection device which can detect troubles in the fibers in high reliability.